1. Field of the Invention
The present invention relates to an image forming apparatus for use in copying machines or the like, and more specifically to a developer sheet feeding device used in the image forming apparatus.
2. Discussion of the Related Art
In conventional copying machines, a developer sheet is fed from a feed unit to a pressing and developing unit, and the developer sheet and a photosensitive recording medium on which a latent image is formed via an exposure unit are superposed together and pressed in the pressing and developing unit, whereby an image is transferred and developed on the developer sheet. Thereafter, the developer paper is fed to an ejecting unit via a heat fixing unit, and the image on the developer sheet is heated and fixed in the heat fixing unit.
In the copying machine of this kind, a first carrier means consisting of a plurality of rollers or the like for carrying the developer sheet from the feed unit to the pressing and developing unit and a second carrier means consisting of a plurality of rollers or the like for carrying the developer sheet from the heat fixing unit to the ejecting unit are driven by one and the same drive source, that is, a motor or the like so that the carrying period of the developer sheet from the feeding of the sheet to the ejection of the sheet via transfer, development and heat fixing is maintained constant.
In the conventional copying machines, however, light radiated on a photosensitive recording medium in the exposure unit becomes weak in an enlarged mode, and, therefore, the carrying speed of the photosensitive recording medium is set at a low speed. Accordingly, the carrying speed of the developer sheet is also set at a low speed so as to meet the carrying speed of the photosensitive recording medium. Accordingly, in the conventional machine in which the carrying time of the developer sheet from feeding to ejection via the transfer, development and heat fixing is maintained constant as described above, the lengthy time required for image formation in an enlarged mode or the like created a problem.
The heat fixing of a developer sheet is carried out at a temperature of from approximately 120.degree. C. to 140.degree. C. Glossiness and color development of an output image depend on the heat fixing temperature and heating time. In the past, therefore, the heat fixing temperature in an equal mode is set and controlled at about 140.degree. C. whereas the heat fixing temperature in the enlarged mode is set and controlled at about 120.degree. C. which is lower than that of the equal mode. The different temperatures maintain the constant glossiness and color development of the output image in the enlarged mode at which the carrying speed of the developer sheet is set at a low speed and in the equal mode at which the carrying speed is set at a high speed. Consequently, the temperature control becomes very cumbersome.